Sudden death during the first year of life is a leading cause of infant mortality in developed countries. In neonates and infants, sudden unexplained death is classified as the sudden infant death syndrome (SIDS) when rigorous efforts to identify the cause of death including a forensic examination are unrevealing. Cardiac mechanisms including life-threatening arrhythmias are suspected to cause an undefined proportion of SIDS and recent evidence indicates that mutations in genes responsible for the congenital long QT syndrome (LQTS) are found in a significant number of cases. Molecular evidence for a link between SIDS and neonatal LOTS supports earlier observations that there is an increased risk of SIDS in infants with a QTc >440 msec based on ECG measurements in 34,000 neonates. Additional anecdotal evidence indicates that LQTS may also present as intrauterine fetal death (IUFD). These observations emphasize the need to more fully understand the prevalence and consequences of arrhythmia-promoting genetic factors in the setting of fetal, neonatal and infant mortality. The goal of this research proposal is to advance our understanding of the genetic risks influencing susceptibility to sudden death before age 1 year. In Specific Aim 1, we will test the hypothesis that mutations in arrhythmia susceptibility genes occur in a measurable subset of SIDS victims and unexplained intrauterine fetal death in late gestation. Separately, we will determine the molecular basis for prolonged QTc interval incidentally discovered in neonates through a massive, ongoing prospective ECG screening trial conducted in Italy. The complete coding regions and splice site sequences of KCNQ1, KCNH2, SCN5A, KCNE1, KCNE2 and coding exons of other candidate genes will be surveyed for variants in four populations, two large SIDS cohorts, a series of fetal deaths, and Italian neonatal LQTS cases identified by prospective ECG screening. In Specific Aim 2, all mutations and rare variants in arrhythmia susceptibility genes will be analyzed for their functional and/or biochemical consequences. We hypothesize that a significant proportion of rare variants in these cases will cause dysfunction of the involved ion channel consistent with impaired myocardial repolarization. Finally, in Specific Aim 3, we will test the hypothesis that unequal expression of SCN5A alleles occurs in SIDS victims and is another potential genetic mechanism (allelic imbalance) that could contribute to the pathological impact of mutations and rare variants. Results from this study will have implications for diagnosis, treatment and prevention of LQTS, SIDS and premature fetal loss. Relevance to Public Health - Sudden unexplained death is a societal burden and vexing clinical problem affecting all age groups. SIDS remains a major reason for infant mortality in the Western world. Identifying factors, including genetic, that contribute to sudden unexpected death in infants has great importance for diagnosis and treatment of preventable causes of SIDS.